1. Field of the Invention
The present invention relates to integrated circuits. More specifically, the present invention relates to methods for determining low threshold voltage V_il and high threshold voltage V_ih.
2. Discussion of Related Art
Due to advancing semiconductor processing technology, each new generation of integrated circuits has greatly increased in speed, functionality and complexity as compared to previous generations. However, each new generation of ICs must also undergo intensive testing and characterization to determine the operating parameters of the ICs. Important operating parameters are usually derived from the voltage transfer curve of the various components of the IC.
The voltage transfer curve shows the voltage level of an output voltage signal V_o of a test circuit for various voltage levels of an input voltage signal V_i. For example, as shown in FIG. 1(a), input voltage signal V_i is applied on the input terminal of an inverter 110. Inverter 110 drives output voltage signal V_o on the output terminal of inverter 110. FIG. 1(b) shows a voltage transfer curve 120 of inverter 110. The voltage range for inverter 110 is from a logic low voltage level V_ol to a logic high voltage level V_oh. Ideally, inverter 110 drives output voltage signal V_o to logic high voltage level V_oh when input voltage signal V_i is equal to logic low voltage level V_ol. Conversely, inverter 110 drives output voltage signal V_o to logic low voltage level V_ol when input voltage signal V_i is equal to logic high voltage level V_oh. Typically, voltage transfer curve 120 is derived by driving input voltage signal V_i from low voltage level V_ol to high voltage level V_oh at specified increments and capturing the voltage level of output voltage signal V_o at each increment.
In actual application, input voltage signal V_i may be slightly higher than logic low voltage level V_ol or slightly lower than logic high voltage level V_oh. Thus, ICs are designed to treat any voltage level between logic low voltage level V_ol and a low threshold voltage level V_il as logic low. Similarly, ICs are designed to treat any voltage level between logic high voltage level V_oh and a high threshold voltage level V_ih as logic high.
Thus, the range of acceptable logic low voltage levels is logic low voltage level V_ol to low threshold voltage level V_il, inclusive. Similarly, the range of acceptable logic high voltage levels is high threshold voltage level V_ih to logic high voltage level V_oh, inclusive. As shown in FIG. 1(b) for inverter 110, low threshold voltage level V_il is equal to the voltage level of input voltage signal V_i where voltage transfer curve 120 first has a slope of negative one (xe2x88x921). High threshold voltage level V_ol is equal to the voltage level of input voltage signal V_i where voltage transfer curve 120 next has a slope of negative one (xe2x88x921) (determining the slope is described in more detail below).
When a new generation of ICs is produced, operating parameters, such as low threshold voltage levels V_il and high threshold voltage level V_ol must be provided to engineers who will use the new IC. Conventional methods to determine low threshold voltage level V_il and high threshold voltage level V_ih are time consuming and inaccurate. For example, a common way to determine low threshold voltage levels V_il and high threshold voltage V_ih is to either simulate the IC using a simulator, such as HSPICE, or use a voltage tester to graph a voltage transfer curve of the IC. With either a simulator or a voltage tester, the voltage transfer curve is typically determined using a point by point process by setting the voltage level of input voltage signal V_i equal to low voltage level V_ol and then slowly incrementing the voltage level of input voltage signal V_i until the voltage level of input voltage signal V_i equals or exceeds high voltage level V_oh. Thus, the voltage transfer curve is drawn using a large set of data points, rather than, using an equation. Without an equation, the mathematical derivative of the voltage transform curve can not be used to determine the slope of the voltage transfer curve. Thus, a test engineer determines the voltage at which the voltage transfer curve has a slope of negative one (xe2x88x921) by visual inspection of the voltage transfer graph. To do this, the test engineer must select the graphing parameters so that input voltage signal V_i and output voltage signal V_o are graphed using the same scale. However, the resulting voltage levels determined by the test engineer is not very accurate due to the ad hoc nature of the measurement. Furthermore, the visual display of the voltage transfer curve may vary depending on the scale used in the graph of the voltage transfer curve which may cause further inaccuracies of the measured voltage transfer curve. Hence, there is a need for a method to accurately determine low threshold voltage level V_il and high threshold voltage level V_ih for integrated circuits.
The present invention uses transform circuits to transform a voltage transfer curve to a transformed voltage transfer curve. Rather than measuring the slope of the voltage transfer curve, low threshold voltage levels V_il and high threshold voltage level V_ih can be determined by finding the maximum and minimum points of the transformed voltage transfer curve, respectively.
Specifically, in accordance with some embodiments of the present invention, a transform circuit transforms the voltage transfer curve of a test circuit to create a transformed voltage transfer curve. The maximum and minimum points on the voltage transform curve are determined to calculate transformed voltage thresholds. The transformed voltage thresholds are transformed into the desired voltage threshold by reversing the transformation used to create the transformed voltage transfer curve.
In a specific embodiment of the present invention, the transformed voltage transfer curve is formed by rotating the voltage transfer curve by 45 degrees clockwise. Thus, the transformed voltage thresholds can be calculated by rotating 45 degrees clockwise. The present invention will be more fully understood in view of the following description and drawings.